Purpose :
Kif17, a kinesin-2 family member, exhibits “tipping”, whereby it accumulates at the distal tip of the axoneme in primary cilia. Kif7, a kinesin-4 family member, additionally localizes to cilia tips, where it controls axoneme structure. We have previously shown that morpholino knockdown of kif17 causes disruption of OS formation at 72hpf. We hypothesize that genetic loss of either kif7, kif17, or both will negatively affect early development of the photoreceptor outer segment (OS).

Methods :
TALENs were used to generate an 11bp deletion allele of kif17, while the CRISPR-Cas9 system was utilized to generate both a 74bp deletion allele and a 32bp insertion allele in kif7. Plastic histology was used to quantify both size and number of OS in control retinas compared to kif7-/- retinas, kif17-/- retinas, and compound kif7-/-; kif17-/- retinas. A qPCR panel of each zebrafish opsin (opn1sw1, opn1sw2, opn1mw2, opn1lw1, rho) was used to monitor each photoreceptor sub-type: UV cone, blue cone, green cone, red cone, and rod respectively.

Results :
Plastic histology revealed that loss of either kif7 or kif17 led to reduced OS number and size in the central, but not ventral, retina at 72hpf. This effect was similar to that observed previously for morpholino knockdown of kif17. Loss of both kif7 and kif17 did not have an additive effect on OS morphology at 72hpf compared to loss of kif17 alone. However, normal OS number and size recovered by 6dpf. qPCR reveals markedly decreased expression of opn1sw1 in mutants compared to control embryos.

Conclusions :
There is a developmental delay in OS formation in either kif7 or kif17 mutant fish. Similar to kif17 morphants, there were defects in OS morphogenesis prior to 3dpf. However, this delay is resolved morphologically by 6dpf. While OS achieve normal size, opn1sw1 expression remains abnormal, indicating that normal OS size can be achieved despite aberrations in opsin expression, particularly in UV cones. Work on the homeostatic mechanisms that control OS size despite altered expression of their principal gene products is ongoing.

This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.